Method for producing devices having piezoelectric films

ABSTRACT

A method for achieving improved piezoelectric films for use in a resonator device is disclosed. The method is based on applicant&#39;s recognition that the texture of a piezoelectric film (e.g., as used in a piezoelectric resonator) is directly affected by the surface morphology of the underlying electrode, and additionally, the surface morphology of the electrode is affected by the surface morphology of the underlying oxide layer or Bragg stack. Accordingly, the invention includes a method of making a device having a piezoelectric film and electrode including controlling the deposition and surface roughness of the electrode and optionally, the Bragg stack.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/502,868, filed by John E. Bower et al. on Feb. 11, 2000 now Pat. No.6,329,305 and entitled “Method For Producing Devices HavingPiezoelectric Films”, which is incorporated herein by reference.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/503,225, titled “Method for Producing Piezoelectric Films withRotating Magnetron Sputtering Process,” filed concomitantly herewith byinventors Barber and Miller and assigned to the present assignee(hereinafter the “Barber application”), which is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to a method for producing electronic devicescontaining a piezoelectric film on a metal electrode. The invention isparticularly usefull in fabricating acoustic resonators andsemiconductor devices.

BACKGROUND OF THE INVENTION

Communications systems typically include a variety of devices (e.g.,filters, mixers, amplifiers, integrated circuits, and so forth).Communications systems are useful for transmitting information (e.g.,voice, video, data) relayed by means of wireless links, twisted pair,optical fibers, and so forth. As wireless communications systems becomemore advanced, signals are being transmitted at higher frequencies(e.g., PCS, ISM, etc). As systems are continually developed in responseto market pressures, the demand for increased performance and reducedsize intensifies. Market forces demand increased integration andreduction of component size.

Resonators such as Bulk Acoustic Wave (BAW) resonators are importantcomponents in the fabrication of bandpass filters and other relatedsemiconductor devices. The BAW resonator is a piezoelectric resonatorthat essentially comprises a film of piezoelectric material (e.g., acrystalline AIN film), deposited between at least two electrodes. Uponapplication of voltage to such a structure, the piezoelectric materialwill vibrate in an allowed vibrational mode at a certain frequency.Piezoelectric resonators are thus useful in discriminating betweensignals based on frequency diversity (e.g., a bandpass filter), and inproviding stable frequency signals (e.g., as in a frequency stabilizingfeedback element in an oscillator circuit).

Typically, the performance of the resonant frequency of thepiezoelectric resonator will depend upon the composition, thickness, andorientation of the piezoelectric material. The resonant frequency of apiezoelectric material is typically inversely proportional to itsthickness; thus, for piezoelectric resonators to operate at highfrequencies {e.g., frequencies greater than ˜700 Megahertz (MHz)}, thethickness of the piezoelectric film must be reduced to a thin film(e.g., having a thickness ranging from about 500 nm to about 10 μm). Theperformance of a piezoelectric resonator is dependent on the crystallineorientation of the atoms comprising the piezoelectric film. The inducedstrain (i.e., stress wave) in a piezoelectric film in response toapplied voltage (i.e., electric field) can only occur from theadvantageous alignment of the atomic dipoles within the piezoelectricfilm. An example of an advantageous film orientation is <002> of AINperpendicular to the substrate. A pulse DC sputtering method fordepositing thin films of piezoelectric materials such as aluminumnitride (AIN) is described in U.S. patent application Ser. No.09/145,323, to Miller et al., “Pulse DC Reactive Sputtering Method forFabricating Piezoelectric Resonators,” filed Sep. 1, 1998, assigned tothe present assignee and incorporated herein by reference. In Miller etal., the quality of the piezoelectric films is improved with thetechniques used to deposit the films themselves.

As may be appreciated, those in the field of communications systems andcomponents continue to search for new methods for increasing systemperformance and integration. In particular, it would be advantageous toprovide new methods for improving the quality of piezoelectric films.These and further advantages of this invention may appear more fullyupon considering the detailed description given below.

SUMMARY OF THE INVENTION

Summarily described, the invention embraces a method for achievingimproved piezoelectric films for use in electronic devices, particularlyresonators. The method is based on applicant's recognition that thetexture of a piezoelectric film (e.g. as used in a piezoelectricresonator) is directly affected by the surface morphology of theunderlying metal layer (e.g., electrode). Accordingly, the inventioncomprises a method of making a device having a piezoelectric film andelectrode comprising controlling the deposition and surface roughness ofthe metal layer, ie., a lower surface roughness for the electroderesults in an improved quality for the piezoelectric film. A low surfaceroughness may be achieved by depositing the metal layer such that it hasa full width, half maximum (FWHM) rocking curve of less than about 4.5°,which produces a piezoelectric layer deposited thereon having a FWHMrocking curve of less than about 3.5°.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, an exemplary embodiment isdescribed below, considered together with the accompanying drawings, inwhich:

FIG. 1 is a perspective schematic illustration of an acoustic resonator;

FIG. 2 is a graph plotting the rocking curves of AIN piezoelectric filmsas a function of rocking curves of Al bottom electrode, showing theeffect of Al texture on AIN texture;

FIG. 3 is a block diagram showing steps for performing the inventivemethod.

It is to be understood that these drawings are for the purposes ofillustrating the concepts of the invention and except for the graph ofFIG. 2 are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

Although specific features and configurations are discussed below, itshould be understood that these examples are for purposes ofillustration only. One skilled in the relevant art will recognize thatother steps, configurations and arrangements may be used withoutdeparting from the spirit and scope of the invention.

FIG. 1 is a perspective schematic illustration of an acoustic resonator,which may be fabricated using the inventive method. The resonator 100comprises a substrate 110, a layer 120 of piezoelectric material, and anacoustic reflecting region 125, such as a Bragg reflecting region,between the substrate 110 and layer 120. Alternative to the reflectingregion 125, a layer of air (not shown) may be used to suspend the layer120 above the substrate 110. A bottom electrode 135 and top electrode130 are disposed on opposite surfaces of the piezoelectric layer 120.

The layer of piezoelectric material advantageously comprises AIN, butmay be made of any suitable material that has piezoelectric qualitiessufficient for the particular resonator application. Typicalpiezoelectric materials include, for example, quartz, zinc oxide (ZnO),and ceramic materials such as lithium niobate (LiNbO₃), lithiumtantalate (LiTaO₃), paratellurite (TeO₂), and lead titanate zirconate(PZT-SA). The substrate typically is comprised of silicon but may befabricated with other materials such as quartz, sapphire, polysilicon,aerogel, and aluminum oxide (Al₂O₃).

The invention pertains to a method for obtaining improved piezoelectricfilms. Applicants have discovered that the operation of a resonatordevice, and in particular, the piezoelectric film, can be improved bymanipulating the morphology of the electrode surface on which thepiezoelectric film is deposited. Applicants further have discovered thatthe growth of the piezoelectric film (e.g., AIN) is not epitaxial (i.e.,the AIN layer is not grown with lattice matching or coordination withthe electrode material), and the morphology of the electrode surfaceaffects the c-axis orientation and rocking curve of the piezoelectricfilm.

The term “texture” as used herein is intended to describe thecrystallographic alignment of grains in a polycrystalline film wherein“maximum texture” denotes a film having an alignment (orientation) ofgrains centered about a single direction at an angle subtended from(relative to) the growth direction. The texture and thus quality of thepiezoelectric layer can be defined by reference to its “rocking curve.”More specifically, ideally the grains are centered about a singledirection; as mentioned above, the performance of the piezoelectric film(e.g. in a piezoelectric resonator), is dependent upon the crystallineorientation of the atoms comprising the film. Typically, however, therewill be a gaussian distribution of directions about which the grains arecentered. The smaller the distribution, the closer the film is tomaximum texture. The distribution of grain directions may be plotted todefine a peak, and the width of the peak at half its maximum height(full-width at half maximum) (FWHM, i.e., the “rocking curve”, reflectsa value for defining the quality of the film texture. The “rockingcurve” number is thus the figure of merit for the film texture, i.e.,the smaller the distribution, the smaller the rocking curve, and thecloser the film is to maximum texture. The piezoelectric layeradvantageously is formed with FWHM rocking curves of less than 11°, witha more preferred low rocking curve being less than about 3.5°, and morepreferably less than 2.5° (FWHM).

The invention is based on applicant's recognition that the electrodecomposition and deposition may be manipulated to approach a maximumtexture for the piezoelectric films and achieve optimal operation of theresonator. In contrast, previous methods for improving piezoelectricfilm texture and performance have focussed on the composition anddifferent methods of depositing the piezoelectric films themselves.According to the invention, the electrode is prepared (e.g., throughdeposition processing, polishing, or material selection) so that it hasa low surface roughness which results in a piezoelectric film having alow rocking curve and improved quality. Advantageously, the electrodehas a surface roughness of less than about 0.1-100 Å, or morepreferably, of less than 15 Å RMS. Applicants have discovered that thesurface roughness of the electrode can be reduced by minimizing its FWHMrocking curve; a low rocking curve for the electrode results in a lowsurface roughness for the electrode surface. Additionally, the surfaceroughness of the electrode can be addressed by the underlying insulatingor Bragg stack layers; e.g., the lower the RMS value of the oxide layerof the Bragg stack, the lower the rocking curve of the electrode, aswell as the RMS value of the electrode surface. A polished (e.g., CMP)oxide Bragg stack yields a better surface roughness for the metalelectrode than the as-deposited oxide Bragg stack. Applicants have alsodiscovered that the surface roughness of the electrode layer may beimproved by selection of the metals comprising the electrode stack.

Advantageously, with the invention, the electrodes and particularly thebottom electrode 135 may comprise aluminum (Al) or a metal stack usingcollimated (physical flux selector) titanium followed by Al (c-Ti/Al).Previous stacked metal electrodes often have comprised c-Ti/TiN/Al asthe composition of choice. Applicants have surprisingly discovered thatthe surface roughness of the Al surface on which the piezoelectric layeris deposited may be improved by at least a factor of two by eliminatingthe TiN layer from the metal stack. Additionally, besides use of Aland/or c-Ti/Al, any other metal having a sufficiently low sheetresistance and low surface roughness may be used for fabricating theelectrodes 130, 135. A metal having a sheet resistance in the range of0.01 to 100 ohms per square (Ω/□) should be sufficiently low, but morepreferably the sheet resistance is approximately 1 Ω/□. A metal having asurface roughness in the range 0.1 to 100 Å (Angstroms) should besufficiently low, but more preferably the surface roughness isapproximately less than 15 Å.

FIG. 2 is a plot of the AIN rocking curve as a function of the Alrocking curve, e.g., for a resonator where the bottom electrode 135comprises collimated titanium and Al (c-Ti/Al) and the piezoelectriclayer 120 comprises AIN. As can be seen, there is a direct, althoughnon-linear, effect of the Al texture (electrode 135) on the AIN texture(piezoelectric 120). The Al texture thus affects the coupling constant(pole/zero separation) for the resonator. Thus, the electrodes (e.g., Aland c-Ti layers) may be formed with a texture such that they have thesmallest full-width at half maximum (FWHM) rocking curves. For thecollimated Ti layer, a useful texture for a resonator application isachieved when the FWHM is in range of about 0.2° to 9°, and a typical,preferred rocking curve is where the FWHM is about less than 4.5°. Forthe Al layers, a useful texture for this application is achieved whenthe FWHM is in range of about 0.2° to 11°, with a typical, preferredrocking curve being about less than 40° (FWHM). The regions ofpiezoelectric material advantageously are formed with FWHM rockingcurves within the range of about 0.2° to 11°, with a typical, preferredlow rocking curve being less than about 2.5° (FWHM). The low rockingcurve of the electrode results in a low surface roughness for theelectrode, which applicants have discovered improves the quality of thepiezoelectric layer.

With this invention, a low root mean square (RMS) morphology can beachieved for the electrode without having to perform polishing, i.e.,chemical mechanical polishing (CMP), of this layer 120 after it isdeposited. The RMS value reflects a true average, absolute value for thedeviation or difference in the surface morphology from a mean value ofzero, the value of zero reflecting a completely smooth surface. The RMSvalue is defined by the square root of the difference between the meansquare and the square of the mean, or in other words, it is thenormalized average value of the roughness relative to the median of themeasured roughness. Advantageously, a crystallographic metal (e.g., c-Tiand Al) is deposited for the electrode with a single orientation andmaximum texture. For example, the Al electrode can have a <111>orientation parallel to the substrate normal. The c-titanium deposition,when used, preferably produces a film of a single orientation <002>.Since the method of achieving a good texture for the piezoelectric layeraccording to this invention does not depend upon lattice matching, asufficiently low rocking curve for the piezoelectric layer (AIN) may beobtained on various surfaces of sufficiently low surface roughness,e.g., substrates with terminated surfaces of silicon, CMP oxide,c-titanium/aluminum, or other suitable materials. Although the latticeparameters of silicon <100> and aluminum <111> are mismatched to AIN,one can obtain a low rocking curve with AIN on Si or a low rocking curvefor AIN on Al. The invention is thus advantageous for not only improvingthe quality of the piezoelectric layers but increasing the flexibilityin selecting the materials used for fabricating the resonator or otherdevices including the piezoelectric films.

Applicants have further discovered that the resultant surface roughnessof the electrode (e.g. FIG. 1, 135), is affected by the surfacemorphology of its underlying layer, e.g., the acoustic reflecting layers125. For example, when a standard oxide deposited by physical vapordeposition is used, a resulting Al texture of 11 degrees (FWHM) mayresult. However, applying the same conditions for depositing the Allayer, the resultant texture of the Al layer is 4 degrees (FWHM) when aCMP oxide is used. The CMP oxide is automatically substantially smooth(e.g., having a RMS surface roughness of less than 3 Å), while astandard PVD oxide of the last layer of a Bragg stack has a surfaceroughness of about 45 to 60 Å RMS. The lower the RMS value of the oxidelayer of the Bragg stack, the lower the rocking curve of the electrode,as well as the RMS value of the electrode surface. A polished (e.g.,CMP) oxide Bragg stack yields a better texture for the metal electrodethan the as-deposited oxide Bragg stack.

FIG. 3 is a block diagram of exemplary steps for performing theinventive method of achieving a resonator structure having improvedpiezoelectric films. Blocks 1 a, 2 a, 3 and 4 reflect one embodiment ofthe method, and blocks 1 b, 2 b, 3 and 4 an alternative embodiment.According to one embodiment, a first step (block 1 a) comprisesdepositing a textured metal layer of c-titanium on a substrate, e.g.,typically a silicon substrate, having acoustic reflecting layers thereonsuch as a Bragg stack. Preferably, at least the last layer of the Braggstack is polished to achieve the best results for the metal electrodetexture. As described above, applicants have discovered that the textureof the electrode and thus, the piezoelectric layer, may be affected bythe quality of the last layer of the Bragg stack. The co-pending Barberapplication, previously cited and incorporated herein, describes anadvantageous method for depositing Bragg stack layers (e.g., comprisingSiO₂ films) and optimizing the deposition process.

The collimated-titanium deposition advantageously results in a film of asingle orientation <002> and a texture having a rocking curve of lessthan 4.5 degrees FWHM. The c-Ti layer may have a thickness in the rangeof about 100 to 1000 Å, and more preferably a thickness of about 300 Å.Deposition techniques known in the field may be used to deposit thislayer, although an advantageous process comprises sputter deposition ata temperature of about 250° C. and deposition rate within the range ofabout 8-12 Å/sec, more preferably at the rate of about 10 Å/sec.

A second step of this exemplary method (block 2 a) comprises depositingAl on the c-Ti layer, without a vacuum break and at a sufficiently highrate and substrate temperature so that the resultant Al texture has aFWHM of less than 4.5°. The Al layer may comprise other alloyingelements, such as Cu, preferably at small percentages. For example, alayer of Al −0.5%Cu is suitable. Deposition parameters may be selectedbased on knowledge of one skilled in the field. Suitable parametersinclude a deposition temperature of 200° C. and rate of about 100 Å/secto produce an Al layer having a thickness in the range of about 200 Å to2500 Å with a nominal thickness of about 1200 Å. No post processing orre-deposition of the Al surface is necessary to achieve the texturedgrowth of the AIN.

Once the stacked metal electrode is deposited, a third step (block 3)comprises patterning the metal electrode. Standard photolithographicprocesses may be used to pattern the electrode. Applying a fourth step(block 4), the piezoelectric layer 120, which advantageously comprisesAIN, is deposited on the patterned electrode. The piezoelectric layer120 advantageously is sputter deposited on the bottom electrode 135,applying the DC reactive sputtering process of Miller et al., citedabove. However, other deposition techniques may be used as well. Forexample, the co-pending Barber application, incorporated herein,describes steps for optimizing the process of depositing piezoelectricfilms using rotating magnet magnetrons and pulsed DC power supplies.

An alternative method for preparing an inventive resonator device isshown in blocks 1 b, 2 b, 3 and 4 of FIG. 3. In this embodiment, theelectrode comprises a single electrode material, rather than the stackedelectrode of the previous embodiment. A first step (block 1 b) comprisesdepositing the metal electrode material, e.g., of Al or any other metalhaving a low surface resistance, on the substrate containing the Braggstack or other acoustic membrane support layer (e.g., SiN_(x)).Deposition parameters known in the field and described above may beapplied to deposit this layer. A second step (block 2 b) comprisespolishing the surface of the electrode layer to the lowest root meansquare figure. This figure preferably will in the range of about 3-10 Å,and more preferably will be less than about 7.5 Å. As in the previousembodiment, the electrode material is patterned (block 3), and the AINlayer is deposited on the patterned metal electrode surface (block 4) toresult in a maximum textured AIN film. Other depositional processparameters can be optimized as described in Miller et al., cited above.

It is understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the appended claims.

We claim:
 1. A method for fabricating a resonator device having apiezoelectric material deposited on at least one metal layer, the methodcomprising depositing the at least one metal layer on a Bragg stackdeposited on a substrate so that the metal layer has a full-width halfmaximum rocking curve of less than about 4.5 degrees and depositing thepiezoelectric material on the metal layer, whereby the texture of thepiezoelectric material has a full-width half maximum rocking curve ofless than about 3.5 degrees.
 2. A method for fabrication a resonatordevice comprising the steps of: depositing a Bragg stack on a substrate;polishing the upper surface of the Bragg stack; depositing a texturedmetal layer on the Bragg stack whereby the metal layer has a surfaceroughness of less than about 15 Å; patterning the textured metal layerto defined a first electrode; depositing a piezoelectric material on theelectrode by sputtering whereby the texture of the piezoelectricmaterial is determined by the full-width half maximum rocking curve ofthe textured metal layer without lattice matching. depositing a metal onthe piezoelectric material to defined a second electrode.
 3. The methodof claim 2 in which the low surface roughness of the metal layer isachieved with the full width half maximum rocking curve of the metallayer being in the range of about 0.2 to 11°.
 4. The method of claim 2in which the upper surface of the Bragg stack is polished so that it hasa root-mean-square surface roughness of less than about 3 Å.